Personally, the customisability without the effort. You get a hundred bells and whistles to make your phone look and feel unique, things that stock android should really provide but doesn't.

People want to interact with their devices in ways that fall outside the blessed functions permitted by the corporate designed pattern deemed appropriate for them. If corporate UI works for you, that's great for you, but it doesn't for everyone.

Consistency between devices. Android isn't android, everyone mucks with it, especially Samsung.

Aside from functionality, stock launchers are more often than not inperformant and battery hungry piles of crap.

I add an extra row and column of apps on the home screen, and make the icons bigger so there's less space between. And bump up the text size since my eyes are getting older.

I'm with you, I never heard of nova launcher until now, and not sure why I would want it.